This file contains brief information about the SCSI tape driver. Last modified: Sun Apr 21 21:19:22 1996 by root@kai.makisara.fi BASICS The driver is generic, i.e., it does not contain any code tailored to any specific tape drive. The tape parameters can be specified with one of the following three methods: 1. Each user can specify the tape parameters he/she wants to use directly with ioctls. This is administratively a very simple and flexible method and applicable to single-user workstations. However, in a multiuser environment the next user finds the tape parameters in state the previous user left them. 2. The system manager (root) can define default values for some tape parameters, like block size and density using the MTSETDRVBUFFER ioctl. These parameters can be programmed to come into effect either when a new tape is loaded into the drive or if writing begins at the beginning of the tape. The second method is applicable if the tape drive performs auto-detection of the tape format well (like some QIC-drives). The result is that any tape can be read, writing can be continued using existing format, and the default format is used if the tape is rewritten from the beginning (or a new tape is written for the first time). The first method is applicable if the drive does not perform auto-detection well enough and there is a single "sensible" mode for the device. An example is a DAT drive that is used only in variable block mode (I don't know if this is sensible or not :-). The user can override the parameters defined by the system manager. The changes persist until the defaults again come into effect. 3. Up to four modes can be defined and selected using the minor number (bits 5 and 6). Mode 0 corresponds to the defaults discussed above. Additional modes are dormant until they are defined by the system manager (root). When specification of a new mode is started, the configuration of mode 0 is used to provide a starting point for definition of the new mode. Using the modes allows the system manager to give the users choices over some of the buffering parameters not directly accessible to the users (buffered and asynchronous writes). The modes also allow choices between formats in multi-tape operations (the explicitly overridden parameters are reset when a new tape is loaded). If more than one mode is used, all modes should contain definitions for the same set of parameters. Many Unices contain internal tables that associate different modes to supported devices. The Linux SCSI tape driver does not contain such tables (and will not do that in future). Instead of that, a utility program can be made that fetches the inquiry data sent by the device, scans its database, and sets up the modes using the ioctls. Another alternative is to make a small script that uses mt to set the defaults tailored to the system. The driver supports fixed and variable block size (within buffer limits). Both the auto-rewind (minor equals device number) and non-rewind devices (minor is 128 + device number) are implemented. By default the driver writes one filemark when the device is closed after writing and the last operation has been a write. Two filemarks can be optionally written. In both cases end of data is signified by returning zero bytes for two consecutive reads. The compile options are defined in the file linux/drivers/scsi/st_options.h. BUFFERING The driver uses tape buffers allocated either at system initialization or at run-time when needed. One buffer is used for each open tape device. The size of the buffers is selectable at compile and/or boot time. The buffers are used to store the data being transferred to/from the SCSI adapter. The following buffering options are selectable at compile time and/or at run time (via ioctl): Buffering of data across write calls in fixed block mode (define ST_BUFFER_WRITES). This should be disabled if reliable detection of end of medium (EOM) for fixed block mode is desired. Asynchronous writing. Writing the buffer contents to the tape is started and the write call returns immediately. The status is checked at the next tape operation. Should not used if reliable EOM detection is desired. Read ahead for fixed block mode (ST_READ_AHEAD). Filling the buffer is attempted even if the user does not want to get all of the data at this read command. Should be disabled for those drives that don't like a filemark to truncate a read request or that don't like backspacing. The buffer size is defined (in 1024 byte units) by ST_BUFFER_BLOCKS or at boot time. If this size is not enough, the driver tries to allocate a large enough temporary buffer that is released when the device is closed. The maximum buffer size is defined by the kernel memory allocation (currently 256 kB for Alphas and 128 kB for other architectures). Allocation of the buffers is done at run-time when they are needed. Allocation of the specified number of buffers can be done at initialization if ST_RUNTIME_BUFFERS is defined non-zero. The advantage of run-time allocation is that memory is not wasted for buffers not being used. The disadvantage is that there may not be memory available at the time when a buffer is needed for the first time (once a buffer is allocated, it is not released). The maximum number of buffers allocated at initialization is defined by ST_MAX_BUFFERS. One buffer is allocated for each drive detected when the driver is initialized up to the maximum. The minimum number of allocated buffers is ST_EXTRA_DEVS (in hosts.h). This ensures some functionality also for the drives found after tape driver initialization (a SCSI adapter driver is loaded as a module). The default for ST_EXTRA_DEVS is two. The driver tries to allocate new buffers at run-time if necessary. The threshold for triggering asynchronous write in fixed block mode is defined by ST_WRITE_THRESHOLD. This may be optimized for each use pattern. The default triggers asynchronous write after three default sized writes (10 kB) from tar. BOOT TIME CONFIGURATION The buffer size, write threshold, and the maximum number of allocated buffers are configurable at boot time using, e.g., the LILO command line. The option syntax is the following: st=aa[,bb[,cc]] where aa is the buffer size in 1024 byte units bb is the write threshold in 1024 byte units cc is the maximum number of tape buffers to allocate (the number of buffers is bounded also by the number of drives detected) IOCTLS The tape is positioned and the drive parameters are set with ioctls defined in mtio.h The tape control program 'mt' uses these ioctls. Try to find an mt that supports all of the Linux SCSI tape ioctls and opens the device for writing if the tape contents will be modified (look for a package mt-st* from the Linux ftp sites; the GNU mt does not open for writing for, e.g., erase). The supported ioctls are: The following use the structure mtop: MTFSF Space forward over count filemarks. Tape positioned after filemark. MTFSFM As above but tape positioned before filemark. MTBSF Space backward over count filemarks. Tape positioned before filemark. MTBSFM As above but ape positioned after filemark. MTFSR Space forward over count records. MTBSR Space backward over count records. MTFSS Space forward over count setmarks. MTBSS Space backward over count setmarks. MTWEOF Write count filemarks. MTWSM Write count setmarks. MTREW Rewind tape. MTOFFL Set device off line (often rewind plus eject). MTNOP Do nothing except flush the buffers. MTRETEN Re-tension tape. MTEOM Space to end of recorded data. MTERASE Erase tape. MTSEEK Seek to tape block count. Uses Tandberg-compatible seek (QFA) for SCSI-1 drives and SCSI-2 seek for SCSI-2 drives. The file and block numbers in the status are not valid after a seek. MTSETBLK Set the drive block size. Setting to zero sets the drive into variable block mode (if applicable). MTSETDENSITY Sets the drive density code to arg. See drive documentation for available codes. MTLOCK and MTUNLOCK Explicitly lock/unlock the tape drive door. MTLOAD and MTUNLOAD Explicitly load and unload the tape. MTCOMPRESSION Sets compressing or uncompressing drive mode using the SCSI mode page 15. Note that some drives other methods for control of compression. Some drives (like the Exabytes) use density codes for compression control. Some drives use another mode page but this page has not been implemented in the driver. MTSETDRVBUFFER Is used for several purposes. The command is obtained from count with mask MT_SET_OPTIONS, the low order bits are used as argument. This command is only allowed for the superuser (root). The subcommands are: 0 The drive buffer option is set to the argument. Zero means no buffering. MT_ST_BOOLEANS Sets the buffering options. The bits are the new states (enabled/disabled) the following options (in the parenthesis is specified whether the option is global or can be specified differently for each mode): MT_ST_BUFFER_WRITES write buffering (mode) MT_ST_ASYNC_WRITES asynchronous writes (mode) MT_ST_READ_AHEAD read ahead (mode) MT_ST_TWO_FM writing of two filemarks (global) MT_ST_FAST_EOM using the SCSI spacing to EOD (global) MT_ST_AUTO_LOCK automatic locking of the drive door (global) MT_ST_DEF_WRITES the defaults are meant only for writes (mode) MT_ST_CAN_BSR backspacing over records can be used for repositioning the tape (global) MT_ST_NO_BLKLIMS the driver does not ask the block limits from the drive (block size can be changed only to variable) (global) MT_ST_DEBUGGING debugging (global; debugging must be compiled into the driver) MT_ST_SETBOOLEANS MT_ST_CLEARBOOLEANS Sets or clears the option bits. MT_ST_WRITE_THRESHOLD Sets the write threshold for this device to kilobytes specified by the lowest bits. MT_ST_DEF_BLKSIZE Defines the default block size set automatically. Value 0xffffff means that the default is not used any more. MT_ST_DEF_DENSITY MT_ST_DEF_DRVBUFFER MT_ST_DEF_COMPRESSION Used to set or clear the density (8 bits), drive buffer state (3 bits), and compression (single bit). If the value is MT_ST_CLEAR_DEFAULT (0xfffff), the default will not be used any more. Otherwise the lower-most bits of the value contain the new value of the parameter. The following ioctl uses the structure mtpos: MTIOCPOS Reads the current position from the drive. Uses Tandberg-compatible QFA for SCSI-1 drives and the SCSI-2 command for the SCSI-2 drives. The following ioctl uses the structure mtget to return the status: MTIOCGET Returns some status information. The file number and block number within file are returned. The block is -1 when it can't be determined (e.g., after MTBSF). The drive type is either MTISSCSI1 or MTISSCSI2. The number of recovered errors since the previous status call is stored in the lower word of the field mt_erreg. The current block size and the density code are stored in the field mt_dsreg (shifts for the subfields are MT_ST_BLKSIZE_SHIFT and MT_ST_DENSITY_SHIFT). The GMT_xxx status bits reflect the drive status. GMT_DR_OPEN is set if there is no tape in the drive. GMT_EOD means either end of recorded data or end of tape. GMT_EOT means end of tape. MISCELLANEOUS COMPILE OPTIONS The recovered write errors are considered fatal if ST_RECOVERED_WRITE_FATAL is defined. The maximum number of tape devices is determined by the define ST_MAX_TAPES. If more tapes are detected at driver initialization, the maximum is adjusted accordingly. Immediate return from tape positioning SCSI commands can be enabled by defining ST_NOWAIT. The MTEOM command is by default implemented as spacing over 32767 filemarks. With this method the file number in the status is correct. The user can request using direct spacing to EOD by setting ST_FAST_EOM 1 (or using the MT_ST_OPTIONS ioctl). In this case the file number will be invalid. When using read ahead or buffered writes the position within the file may not be correct after the file is closed (correct position may require backspacing over more than one record). The correct position within file can be obtained if ST_IN_FILE_POS is defined. (The driver always backs over a filemark crossed by read ahead if the user does not request data that far.) Kai M{kisara